The United States Naval Observatory is a shore activity under the command of the Superintendent, U.S. Naval Observatory and the Commander, Naval Meteorology and Oceanography Command. The oldest scientific institution in the U.S. Navy, the Naval Observatory began on 6 December 1830, as a depot for the Navy's navigational charts and instruments. Nautical charts were maintained, and astronomical observations were made to rate the chronometers distributed to Navy vessels. Lieutenant Louis Goldsborough, a veteran of the sea from the age of nine, became the first Officer-in-Charge. He was followed by Lieutenant Charles Wilkes who later gained fame by leading the U.S. Exploring Expedition to the Pacific Ocean and the Antarctic. Lieutenant James M. Gilliss relieved Wilkes in 1837. Gilliss consulted the prominent scientists in this country and abroad, purchased instruments and books in Europe, and personally supervised the construction of a new observatory at 24th and E streets, NW, on a hill overlooking the Potomac River. The construction was completed by October 1844 but, to Gilliss' dismay, Lieutenant Matthew Fontaine Maury was named the first Superintendent. Gilliss had yearned for the job himself.
Highly visible in 1844, with an array of astronomical instruments unparalleled in the country, the U.S. Naval Observatory began to build its worldwide reputation. Maury set to work cataloging all stars that could be seen with the Observatory's instruments, an impossible task that, in the end, was never completed. Maury's tenure was characterized by an emphasis on oceanography, and in 1854 the facility was designated as the U.S. Naval Observatory and Hydrographical Office. Scientific research consisted primarily of determining fundamental celestial positions, motions, and astronomical constants. Systematic observations were made of the Sun, Moon, planets and bright stars.
In January 1846, Maury discovered the breaking up of Biela's Comet into two pieces. In February 1847, Sears Walker found that the planet Neptune, which had been discovered the previous September, was actually the "star" observed by the French astronomer LaLande in 1795. The discovery permitted a more accurate determination of Neptune's orbit. In 1854 James Ferguson, using the 9.6-inch refractor, became the first American to discover a minor planet. Two more minor planets were discovered by Naval Observatory astronomers during the next 6 years. Astronomical observations were published under the name Naval Observatory at first, and later under the name U.S. Naval Observatory.
In 1845, at the request of the Secretary of the Navy, the Observatory installed a time ball atop the 9.6-inch telescope dome. The time ball was dropped every day precisely at Noon, enabling the inhabitants of Washington to set their timepieces. Ships in the Potomac River could also set their clocks before putting to sea. The Observatory's Time Service was initiated in 1865. A time signal was transmitted via telegraph lines to the Navy Department, and also activated the Washington fire bells at 0700, 1200, and 1800. This service was later extended via Western Union telegraph lines to provide accurate time to railroads across the nation. The Observatory participated in a program of determining longitude by comparing local time with that telegraphed from a clock at another fixed observatory, and thus exchanged time signals with other observatories and with the Coast Survey field parties.
In 1861 Maury reluctantly resigned to join the Confederate forces, and Gilliss again took over the Observatory. The Observatory was staggering under the war-induced workload of providing navigational instruments and charts to a Navy responding to Lincoln's call for a blockade of the South. In August 1863, Lincoln sought a moment of solace at the Observatory looking at the Moon through the 9.6-inch telescope.
The post-Civil War years saw the Observatory rapidly becoming one of the world's leading astronomical observatories. Thousands of observations were now in print, thanks to Gilliss, and American astronomers could rely on the Naval Observatory rather than Europe for fundamental star positions. In 1866 the Hydrographic Office was separated from the Observatory and took up quarters in the "Octagon House" in Washington, DC. Seven years later the Observatory installed the largest refracting telescope in the world-the 26-inch-still in use today and now located just west of the present Observatory's main building. In 1874 and 1882, teams were dispatched around the world to record the transit of Venus across the solar disk. In 1876, the Naval Observatory helped the nation celebrate its Centennial, proudly displaying evidence of the Observatory's scientific achievements at the Centennial Exhibition in Philadelphia. In 1877, Asaph Hall discovered the two satellites of Mars with the 26-inch telescope. Simon Newcomb, the Superintendent of the Nautical Almanac Office from 1877 to 1897, was one of the premier American astronomers of the 19th century.
After nearly fifty years at the site on the Potomac River, hampered by fog and deteriorating buildings, in 1893 the U.S. Naval Observatory moved to its present location on Massachusetts Avenue in Northwest Washington, D.C. At that time the site was well outside the city, and separated from it by a deep valley. Three of the buildings (the main building, the 26-inch dome and the transit circle buildings) were designed by the renowned architect Richard Morris Hunt. The Superintendent's residence, located to the north of the Observatory's main building, was also completed at this time. It was designed by Leon Dessez. (In 1929 the Superintendent's residence became the home of the Chief of Naval Operations, and in 1974 Congress designated it as the Temporary Official Residence of the Vice President of the United States.)
As an event that provided an opportunity to rethink old programs and to propose new ones, as well as in the provision of new facilities, the move to the new location was an important landmark in the history of the Observatory. Along with new programs such as the daily monitoring of solar activity with a photoheliograph (1899-1971), the old functions of timekeeping and meridian and equatorial observations were kept intact. The move also provided the occasion for the Nautical Almanac Office, in Cambridge, Massachusetts since 1849 and located in Washington, D.C. since 1866, to become officially a part of the Naval Observatory.
The challenge was now to achieve greater and greater accuracy in all areas of its mission, a quest that characterizes much of the research at the U.S. Naval Observatory during the twentieth century. Greater accuracy required improved technology, and nowhere was this more evident than in the determination, maintenance and dissemination of time. Beginning in 1934, the Observatory determined time with a photographic zenith tube (PZT), a specialized instrument that points straight upward toward the zenith and automatically photographs selected stars crossing the zenith. This gave a measure of the Greenwich Mean Time (now called Universal Time), the "time of day" based on the rotation of the Earth. Improvements in clock technology, including the Shortt free-pendulum clock and quartz crystal clocks, soon proved conclusively that the Earth's rotation was not uniform, and a new uniform time scale known as Ephemeris time came into use in 1956.
Defined by the orbital motion of the Earth about the Sun, in practice Ephemeris time was determined by observations of the Moon, first undertaken with the dual rate moon camera, invented by William Markowitz at the Naval Observatory in 1951. In 1984 the family of time scales known as dynamical time replaced Ephemeris time as the time based on the motion of celestial bodies according to the theory of gravitation, now taking relativistic effects into account. In the meantime, the development of atomic clocks brought about the introduction of a much more accessible time - the Atomic time scale based on the vibration (an energy level transition) of the cesium atom.
In 1958 the Naval Observatory and Britain's National Physical Laboratory published the results of joint experiments that defined the relation between Atomic time and Ephemeris time. (An interesting scientific and philosophical question is whether the relationship between Atomic time and gravitational time remains constant.) Since 1967 the international definition of the second has been based on these joint experiments. Atomic time is kept synchronized with universal time by the addition or subtraction of a leap second whenever necessary.
Time dissemination has also been continuously improved. In 1904 a naval radio station transmitted the first radio time signals ever; they were derived from a U.S. Naval Observatory clock. This was the beginning of a system of radio time, constantly improved and increasingly automated through the century, that now spans the globe. The function of rating, repairing and disseminating chronometers and other nautical instruments, a major and especially critical effort during World War II , was transferred from the Observatory to the Optical Section of the Norfolk Naval Shipyard in Portsmouth, Virginia in 1950.
The determination of the fundamental celestial coordinate system, against which the motions of all other celestial bodies must be measured, has been carried out during this century at the U.S. Naval Observatory by two transit circle telescopes, the nine-inch (operated from 1894 to 1945) and the six-inch, designed by William Harkness and mounted in 1899. The catalogues produced by these instruments are fundamental in the sense that in addition to selected stars, observations are also made of the Sun, Moon, planets and asteroids. These solar system objects are used to determine the positions of the equator and equinox, which define the orientation of the fundamental celestial coordinate system. The six-inch transit circle has produced six fundamental star catalogues since 1924. This instrument has undergone many changes in technology to improve accuracy, from the method of reading its graduated circle, to a traveling-wire micrometer, digital readouts of the micrometer measures, and computerized data acquisition and telescope control. In 1956 a new seven-inch transit circle was installed to replace the nine-inch. During the late 1960s and early 1970s it was located in El Leoncito, Argentina, for a program of Southern hemisphere observations; after an intensive development program, it was moved to the South Island of New Zealand in 1984.
The quest for greater accuracy with the equatorial telescopes has been carried out by improvements to the 26-inch, as well as by the addition of new specialized telescopes. In 1935 a 40-inch Ritchey-Chretien aplanatic reflecting telescope, one of the first of its kind, was completed by G.W. Ritchey, a pioneer in telescope design who spent four years at the Naval Observatory on this project. (The Hubble Space Telescope is of this design.) This telescope was moved to the newly-established Flagstaff station in Arizona in 1955, and was joined in 1963 by a new 61-inch astrometric reflector, designed and constructed under the direction of Naval Observatory Scientific Director Kaj Strand.
Again, the 61-inch is a pioneering design: the first, the biggest and the most accurate of its kind ever built. Together, the 40-inch and the 61-inch determine the relative positions, brightness, colors, and spectral types of stars with electronic cameras, and by photography and photometry. The 61-inch, which has a focal length of 50 feet, has, since its inception, carried out the worldÕs largest program of determining stellar parallaxes; that is, accurate determinations of distances of nearby stars. For the first 20 years of its existence it has concentrated on stars with magnitudes ranging from 12 to 18. For this program 35 to 40 photographic plates were taken of each star, and the plates were measured with an automatic measuring engine in Washington, the main parts of which are made of massive blocks of granite, and which can determine star positions on photographs to better than one micron.
The 61-inch reflector, together with an eight-inch double astrograph and a 15-inch astrograph, has also been involved in a program on the astrometry of solar system objects. It was with photographs taken with the 61-inch for this program that James W. Christy discovered a satellite of Pluto in 1978, 101 years after Asaph Hall discovered the moons of Mars. That discovery resulted in a precise determination of the mass of Pluto, and raised new speculations about the possibility of a planet beyond Pluto. In the meantime, the 26-inch, the old classical refractor, was engaged throughout the century in a program to observe natural satellites and double stars; almost 30,000 visual measures of double stars were made in this program between 1961 and 1990. Since 1990 double stars have been observed with a technique known as "speckle interferometry". By taking very short exposures with a Charge-Coupled Device (CCD) camera, astronomers can actually use the blurring effect of Earth's atmosphere to their advantage to measure the separations and position angles of double star components. The technique is ideally suited to the 129 year-old optics of the great telescope, and relatively unaffected by the urban location of the Observatory. Several thousand stars are measured annually, and the database of such observations, added to the visual observations dating back over a century, provide for one of the most concise double star catalogs in the world.
Throughout the century, the Nautical Almanac Office has fulfilled its essential function of predicting the positions of celestial bodies. Utilizing transit circle observations from the U.S. Naval Observatory and around the world, the Nautical Almanac Office has improved the theories of the orbital motions of solar system objects. These theories were used to construct ephemerides for astronomers, navigators, and surveyors, printed for most of the century as The American Ephemeris and Nautical Almanac, but since 1981 as The Astronomical Almanac. For marine navigation there is a separate publication, The Nautical Almanac. For the use of air navigators during World War II the Nautical Almanac Office designed and developed the American Air Almanac, first issued in 1941, and still issued as The Air Almanac. Since the beginning of World War II the Nautical Almanac Office has been in the forefront of the development and utilization of computerized techniques in astronomy. This is necessary not only for the production of the Almanacs, and for providing astronomical data of various types for locations worldwide, but also for a wide range of research in celestial mechanics carried out at the Nautical Almanac Office.
The U.S. Naval Observatory continues to be the leading authority in the United States for astronomical and timing data required for such purposes as navigation at sea, on land, and in space, as well as for civil affairs and legal matters. Its current Mission Statement, promulgated in 1984 by the Chief of Naval Operations, reads:
"To determine the positions and motions of celestial bodies, the motions of the Earth, and precise time. To provide the astronomical and timing data required by the Navy and other components of the Department of Defense for navigation, precise positioning, and command, control, and communications. To make these data available to other government agencies and to the general public. To conduct relevant research; and to perform such other functions or tasks as may be directed by higher authority."
The U.S. Naval Observatory, via its Directorates for Astrometry and Time, carries out its primary functions by making regular observations of the Sun, Moon, planets, selected stars, and other celestial bodies to determine their positions and motions; by deriving precise time interval (frequency), both atomic and astronomical, and managing the distribution of precise time by means of timed navigation and communication transmissions; and by deriving, publishing, and distributing the astronomical data required for accurate navigation, operational support, and fundamental positional astronomy. The U.S. Naval Observatory conducts the research necessary to improve both the accuracy and the methods of determining and providing astronomical and timing data.
In addition to its Washington, DC, headquarters, the U.S. Naval Observatory maintains several field activities. The Time Service Alternate Master Clock Station at Falcon Air Force Base in Colorado serves as a backup to the Master Clock system in Washington, D.C. The Flagstaff Station provides a dark sky site at Flagstaff, Arizona, where the 61-inch astrometric reflector, the 40-inch reflector, a 24-inch reflector, and an 8-inch transit circle telescope are located. The 8-inch astrograph, formerly stationed in Washington, has completed a 2-year survey of the southern hemisphere skies from the Cerro Tololo Inter-American Observatory (CTIO) in Chile. It is currently completing the northern hemisphere portion of the sky survey from the Flastaff Station.
The transit circle telescopes have now completed their historic mission of determining the fundamental celestial coordinate system. In their place the Navy Prototype Optical Interferometer (NPOI) has been constructed at Anderson Mesa near Flagstaff, Arizona. It is a new generation synthetic aperture telescope that will precisely determine the positions of stars to accuracies 100 times better than conventional ground-based techniques, thus providing the necessary reference points for precise guidance and targeting systems, as well as for a variety of astronomical purposes. It consists of two arrays of mirrors, which gather the starlight, which is then combined in such a way that the interference patterns of the waves yield valuable scientific information. The first array, using four fixed mirrors arranged in a Y shape along 20-meter arms, will produce astrometric data, while the second array, using six mirrors movable along the arms of a 250-meter Y, will be used for imaging objects. The astrometric array and the inner part of the imaging array will be completed in late 1995. By 1998 the imaging array was extended to its full size.
The Flagstaff station is at the forefront of electronic techniques in astronomy, using charge-coupled device (CCD) cameras, to measure relative positions of stars and other objects. Accurate stellar colors need to be taken into account in position determination, and are measured at the Flagstaff station. The U. S. Naval Observatory Precision Measuring Microdensitometer (PMM) is being used here to digitize approximately 5000 photographic plates from the Palomar Observatory Sky Surveys I and II. This program will result in a new star catalogue with a billion objects. The Flagstaff station is also working in the field of infrared astronomy, where the sky appears quite different than at optical wavelengths
Highly accurate Earth orientation (rotation rate and polar motion) determinations are now made using radio telescopes that track quasars, powerful sources of radio energy some five to fifteen billion light-years distant. To accomplish this, the Very Long Baseline Interferometry (VLBI) system is used. The VLBI system regularly uses telescopes at the National Radio Astronomy Observatory in Green Bank, West Virginia, as well as sites in Alaska and Hawaii. The VLBI correlator needed to analyze these observations is located at the U.S. Naval Observatory in Washington, D.C. The extragalactic reference frame produced by these observations is now the most accurate celestial coordinate system. Future DoD needs for USNO Earth Orientation information include a GPS requirement for long-range predictions of the Earth's rotation with an error of 3 meters.
By a Department of Defense directive, the U.S. Naval Observatory is charged with maintaining the DoD reference standard for Precise Time and Time Interval (PTTI). The Superintendent is designated as the DoD PTTI Manager. The U.S. Naval Observatory has developed the world's most accurate atomic clock system, accurate to a billionth of a second per day. Increasingly accurate and reliable time information is required in many aspects of military operations. Modern navigation systems depend on the availability and synchronization of highly accurate clocks. This holds for such ground-based systems as LORAN-C as well as for the Department of Defense satellite-based Global Positioning System (GPS). In the communications and the intelligence fields, time synchronized activities are essential. The U.S. Naval Observatory Master Clock is the time and frequency standard for all of these systems. Thus, that clock system must be at least one step ahead of the demands made on its accuracy, and developments planned for the years ahead must be anticipated and supported.
The Master Clock system now incorporates hydrogen masers, which in the short term are more stable than cesium beam atomic clocks, and mercury ion frequency standards, which are more stable in the long run. These represent the most advanced technologies available to date. In the past highly accurate portable atomic clocks have been transported aboard aircraft in order to synchronize the time at Naval Bases and other Department of Defense facilities around the world with the Master Clock. Accurate time synchronization with the Master Clock is now carried out through two way satellite time transfer, or through the use of atomic clocks on GPS satellites, which provide the primary means of time synchronization and worldwide time distribution.
In the production of the Astronomical, Air, and Nautical Almanacs, the U.S. Naval Observatory must accurately predict the positions of stars and planets for several years in the future. In the case of the planets, this prediction requires a very precise knowledge of their orbits, and involves a research effort of formidable magnitude, requiring some of the most accurate mathematical calculations made in any field of science. The planets in turn, through their gravitational force, have an influence on the motion of the Earth, and therefore precise knowledge of planetary masses and positions is essential to accurately predict the future positions of the Earth in space, its motion and orientation. The Astronomical Applications Department, which undertakes this work, also distributes astronomical data by computer. In the past this led to the development of the Almanac for Computers and the Floppy Almanac, both of which have now been superseded by the Multiyear Interactive Computer Almanac (MICA), a 250-year computer-based almanac available for both Windows- and MacOS-based operating systems. A computerized celestial navigation package, known as STELLA (System to Evaluate Latitude and Longitude Astrometrically), is now available to DoD clients. It is important to remember that in the event of war, celestial navigation cannot be jammed.
The increasing demands of the Navy and other components of the Department of Defense for more accurate astronomical and timing data require a continuing, intense effort by the U.S. Naval Observatory in order to adequately carry out its unique mission. The U.S. Naval Observatory, the realization of John Quincy Adams' vision of an American "Lighthouse of the Sky", remains today at the leading edge of technology for astrometric and timing data, and is an institution of which the U.S. Navy is justifiably proud.
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